PROJECT SUMMARY GNE myopathy is a slowly progressive muscle disease resulting in marked disability, including wheelchair use, and ultimately requiring dependent care. Currently, there is no treatment available for GNE myopathy. N-acetyl-D-mannosamine monohydrate (ManNAc), the first committed precursor for the biosynthesis of Neu5Ac (sialic acid), and a substrate of the GNE enzyme defective in GNE myopathy, is being developed for this orphan indication. A first-in- human Phase 1 trial has been completed and an Open-Label Phase 2 trial is ongoing. We propose a randomized, double-blinded, placebo-controlled trial to evaluate the efficacy and safety of ManNAc in subjects with GNE myopathy. We hope that this study will inform future trials and will eventually lead to FDA approval for ManNAc in GNE myopathy. The primary efficacy endpoint and design of our proposed multicenter trial in 50 GNE myopathy subjects is based on knowledge collected for 5 years as part of an ongoing natural history study (NCT01417533; PI: Dr. Nuria Carrillo) which led to the development of the GNE Myopathy Disease Progression Model. This disease progression model is a novel tool that has allowed the design of a trial that overcomes the challenges of determining treatment effect in this rare and slowly progressive disease. The disease progression model facililates recruitment of subjects within a wide range of the disease spectrum, increasing the generalizability of our findings to the patient population. Furthermore, our strategy could be applied to other slowly progressive muscle diseases. Finally, we hope that our proposed trial and patient recruitment efforts will increase awareness of GNE myopathy and identification of undiagnosed patients. 1